This invention relates to the preparation of lenses by solidification of a vapor-phase material in the shape of a lens as opposed to the conventional formation of lenses by such means as grinding and polishing or etching.
The prior art has disclosed numerous methods for preparing lenses including grinding and polishing, etching, and the solidification of a liquid, especially a molten glass or liquid plastic. Each method has significant shortcomings in producing lenses of close tolerances to a particular symmetry and surface figure. To illustrate:
Grinding and polishing operations are hampered by the fact that precise machine or manual control is required to insure that the desired precise curvature is obtained. Additionally, grinding and polishing operations are, in general, not suitable for producing very small lenses or lens arrays.
Etching methods, especially for lens arrays, are heavily dependent upon acid resists being applied to a substrate in a desired accurate pattern, upon differences or gradients in acid resistance of a substrate, upon carefully controlled compositions having particular etching rates, or upon a combination of those three factors.
The fabrication of small lenses ad lens arrays has relied primarily upon molding or upon the curvature developed as a result of surface tension during the solidification of a material from the liquid state, e.g., the cooling of molten glass. Precise control of curvature is very difficult when lenses and lens arrays are formed through surface tension because of inherent limitations in the method. Thus, the lens surface figures obtainable are restricted to those naturally occurring in droplets of the liquid material. Moreover, even if this surface figure is suitable, the lens curvature may vary as a result of variations in surface tension, density of the liquid material, and quantity of the liquid material. Finally, in the case of forming lens arrays, it is difficult to position and retain a liquid at the precise desired location in the exact desired quantity. This last problem is illustrated in U.S. Pat. No. 3,351,449.
That patent describes the deposition of a gob of molten glass of controlled volume onto a substrate. The curved surfaces of the gob upon solidification could be considered to represent a lens. Such methods, however, are limited to materials having a plastic state where they can be rendered workable by exposure to elevated temperatures or by admixture with a solvent. Those methods are very difficult to apply to such desirable optical materials as alumina because of the exceedingly high temperatures required and the extremely rapid rate of solidification exhibited by those materials. Furthermore, the surface figure secured is governed by surface tension forces, which will not necessarily yield the figure required for a particular application and which restrict the symmetry of the lenses to cylindrical, i.e., circular in plan view, which also may not be desired for certain applications.
Some of those problems can be reduced by utilizing molding. Japanese Patent Application 66,115, filed Apr. 14, 1983, discloses the preparation of concave spherical lenses by dropping molten alumina onto a spherical mold. That process, however, requires prior fabrication of the mold, a difficult operation demanding grinding and polishing procedures of the same extreme degree of precision as is necessary in the direct formation of lenses by grinding and polishing. Also, the dropping process inherently results in the upper surface figure of the lens being defined by surface tension forces. Moreover, even if the symmetry and surface figure of the lenses prepared by molding or as developed solely through surface tension forces are suitable, the highest quality lenses and lens arrays with the fewest imperfections and highest polish require further treatment. Finally, the extremely high temperatures required preclude the use of most mold materials except graphite.
Vacuum deposition processes, such as thermal evaporation or sputtering, have been employed to coat lenses or windows with optical materials. The basic mechanism of those processes involves transporting the optical material in the form of vapor along lines of sight in a vacuum from the source of the material to a substrate where it condenses essentially instantaneously to a solid. U.S. Pat. No. 3,846,165 is illustrative of such a process and describes the deposition of such materials as silicon monoxide, alumina, and tungsten oxide to form an anti-reflective coating. Exceedingly close thickness tolerances are demanded for the proper functioning of such coatings. A mask with substantial open area is typically placed in contact or nearly in contact with the surface of the object to be coated to confine the coating deposit to the proper location. The mask serves solely that function and none other. The required extreme degree of thickness uniformity is strictly dependent upon the proper functioning of the evaporative source of the material.
Masking has also been utilized in depositing small narrow patterns, commonly of metals, through vacuum deposition processes. U.S. Pat. No. 4,100,313 describes the use of a long narrow slit of substantial thickness combined with a second thin mask with a single limiting aperture between the thick mask and the source of the coating material. The combination of slit plus object to be coated is tilted relative to the line of sight along which the coating material is being transported from the source of coating material to the object to be coated. That action reduces the effective width of the slit. The apertured thin mask produces a beam of coating material which exhibits reduced angular divergence such that, after passing through the tilted thick mask, it continues along the path of line of sight until being deposited in a narrow line with well-defined boundaries.
U.S. Pat. No. 4,278,710 discloses the use of a similar aperture in a thin mask. Hence, a mask containing an enlarged version of the desired pattern for depositing a coating is placed near the source of the material to be evaporated and the thin mask with a single, small limiting aperture is placed between the pattern mask and the object to be coated. Material emitted from the evaporative source passes through the pattern mask and, after passing through the aperture in the thin mask, has reduced angular divergence as it continues on to the object to be coated, thereby forming a deposit having well-defined edges which is a reduced version of the pattern in the pattern mask.
U.S. Pat. No. 4,273,812 describes the use of a mask separated slightly from the object to be coated, along with two properly placed evaporative sources, to obtain overlap beneath the mask of material deposited through two mask openings.
Whereas the prior art discloses the use of vacuum deposition techniques to transport optical materials and pattern them in a two-dimensional manner, utilizing masks as described above, those processes are not operable for the three-dimensional shape control demanded for the formation of lenses and lens arrays. Most important, the use of apertures to restrict the angular divergence of the material being deposited, as taught in U.S. Pat. No. 4,278,710 and in U.S. Pat. No. 4,100,313, leads to a deposit having an overall shape which, in plan view, is determined by the mask, but which is of essentially uniform thickness with sharp edges and, hence, not operable for use as a lens. U.S. Pat. No. 4,273,812 utilizes a mask spaced from the substrate and does not mention the use of limiting apertures, but employs two evaporative sources of material and narrowly-separated mask openings which yield deposits overlapping one another to produce a continuous, fairly uniform deposit which, likewise, is also unsuitable for use as a lens. Furthermore, those prior art methods provided no means for removing the deposited material from the substrate after their formation.
Therefore, the primary objective of the instant invention was to develop a method for preparing lenses and lens arrays which avoids the above problems of formation.